Background: Histone H3 (H3) phosphorylation plays important roles in mitotic chromosome condensation. We reported that H3 phosphorylation occurs at Ser28, as well as at Ser10 during mitosis, at least in mammals. Aurora B was recently demonstrated to be responsible for Ser10 phosphorylation in S. cerevisiae, C. elegans, Drosophila and Xenopus egg extract.
Results: We compared the distribution of Aurora-B with that of H3 phosphorylation. Aurora-B was primarily localized in the heterochromatin of late G2 phase cells, where only Ser10 phosphorylation was observed. The treatment of such cells with calyculin A induced Ser28 phosphorylation in the Aurora-B-localized area. During prophase to metaphase, Aurora-B was distributed in condensing chromosomes where Ser10 and Ser28 were phosphorylated. Aurora-B can phosphorylate H3-Ser10 and -Ser28 in nucleosomes in vitro. Transfection of a dominant-negative mutant of Aurora-B resulted in a reduction of H3 phosphorylation, not only at Ser10 but also Ser28, during mitosis.
Conclusions: With regard to mitotic chromosome condensation, Aurora-B directly phosphorylated H3, not only at Ser10 but also at Ser28. The level of Ser28 phosphorylation is diminished to undetectable levels by PP1 phosphatase prior to entry into mitosis.